In the above-identified applications, there is disclosed a system for the production of spherical nuclear fuel (fissionable-material) or breeder (fertile-material) particles -- generally designated hereinafter as "nuclear-reactor particles" -- with diameters of several hundred microns for direct use in the cores of power-producing or breeder-type nuclear reactors or for incorporation in fuel or fertile elements used in nuclear reactors. In accordance with the teachings of this application, ball-shaped droplets of an uranium-containing aqueous solution undergo chemical reaction and hardening.
The aqueous solution for the production of uranium dioxide nuclear particles may contain uranyl nitrate, urea and hexamethylenetetramine, produced by the addition of solid methylenetetramine to an aqueous solution of uranyl nitrate and urea-containing more than 500 g of uranium per liter. The droplets are caused to pass downwardly through a liquid in which water is insoluble and which does not solubilize in water, at an elevated temperature, generally above 100.degree. C. After a brief residence time in this liquid, the coherent spherical particles are removed, treated with a highly volatile solvent to remove traces of the organic liquid, and are washed with an ammoniacal solution for removal of excess nitrate and urea. The particles are dried and are then sintered in a reducing atmosphere for several hours at a temperature of 1200.degree. to 1400.degree. C.
A simplified system for this purpose has a nozzle located directly above the free surface of the hot organic liquid so that the droplets of the aqueous solution form at the end of the nozzle and drop into the hot organic liquid.
The spherical uranium dioxide particles with a diameter of several hundred microns can be used in several different types of reactor as fuel or fertile materials. When they are used in so-called "high-temperature reactors" the particles are generally provided with pyrolytic carbon sheaths or coatings (so-called "coated particles") and may be embedded in a graphite matrix. It is important that the spherical particles be of uniform size so that the uranium density throughout the matrix is constant. Thus considerable effort has been invested heretofore in improvements in the particle-production techniques so as to obtain particles of uniform controlled size.
In a publication by Hass and Lackey, Oak Ridge National Laboratory, ORNL-TN-4094, it is taught that spherical particles of oxides of uranium and thorium can be recovered from colloidal solutions by gellation. In these cases, the colloidal solution is dropped from a nozzle surrounded by a sleeve channel into an alcohol stream. The individual droplets are produced by pulsing the aqueous solution in the supply line by a vibrator oscillating in the flow direction. The droplets, turn from the nozzle in the alcohol stream, form spheroids by the action of surface tension and are hardened in this form.
The pulsing stream takes place well ahead of the nozzle so that the droplet size is influenced more by the effect of the alcohol stream in tearing away the colloidal suspension as it emerges from the nozzle than by the pulsations. As a consequence, the uniformity of the droplets may not satisfy many requirements and the sphericity leaves much to be desired.
In both systems, the nozzles are readily blocked, thereby interrupting operation of the apparatus and irregular practical size distributions are produced.
It is the principal object of the present invention to provide an apparatus which is capable of overcoming these disadvantages and particular to provide a droplet-forming dispenser which is not as susceptible to blockage and is capable of producting particles of regular particle size (i.e. droplets).
Another object of this invention is to provide an apparatus for carrying out the process described in the aforementioned patent application, for the production of spherical nuclear reactor particles of uranium dioxide, such that the very droplets used in these systems are of uniform diameter so that side processes for classification and the like are no longer necessary.
Still another object of the invention is to provide an apparatus for producing spherical uranium dioxide particles which avoids the disadvantages of prior-art systems.